In the aforementioned patents, a method is disclosed for producing metal powders comprised of a plurality of constituents mechanically alloyed together such that each of the particles is characterized metallographically by an internal structure in which the starting constituents are mutually interdispersed within each particle. In general, production of such particles involves the dry, intensive, impact milling of powder particles such that the constituents are welded and fractured continuously and repetitively until, in time, the intercomponent spacing of the constituents within the particles can be made very small. When the particles are heated to a diffusion temperature, interdiffusion of the diffusible constituents is effected quite rapidly. The powders produced by mechanical alloying are subsequently consolidated into bulk forms by various well known methods such as degassing and hot compaction followed by shaping, e.g., by extrusion, rolling or forging.
The potential for the use of mechanically alloyed powder is considerable. It affords the possibility of improved properties for known materials and the possibility of alloying materials not possible, for example, by conventional melt techniques. Mechanical alloying has been applied to a wide variety of systems containing, e.g., elemental metals, non-metals, intermetallics, compounds, mixed oxides and combinations thereof. The technique has also been used to enable the production of metal systems in which insoluble non-metallics such as refractory oxides, carbides, nitrides, silicides, and the like can be uniformly dispersed throughout the metal particle. In addition, it is possible to interdisperse within the particle larger amounts of alloying ingredients, such as chromium, aluminum and titanium, which have a propensity to oxidize easily. This permits production of mechanically alloyed powder particles containing any of the metals normally difficult to alloy with another metal. Further, it has been applied to produce alloy systems of readily oxidizable components such as aluminum, magnesium, lithium, titanium, and copper.
The present invention is independent of the type of mill used to achieve the mechanically alloyed powder. However, one aspect of the present invention is that the milling to produce the mechanically alloyed powder is carried out in a "gravity-dependent-type" ball mill. Dry, intensive, high energy milling is not restricted to any type of apparatus. Heretofore, however, the principal method of producing mechanically alloyed powders has been in attritors. An attritor is a high energy ball mill in which the charge media are agitated by an impeller located in the media. In the attritor the ball motion is imparted by action of the impeller. Other types of mills in which high intensity milling can be carried out are gravity-dependent-type ball mills, which are rotating mills in which the axis of rotation of the shell of the apparatus is coincidental with a central axis. The axis of a gravity-dependent-type ball mill (GTBM) is typically horizontal but the mill may be inclined even to where the axis approaches a verticle level. The mill shape is typically circular, but it can be other shapes, for example, conical. Ball motion is imparted by a combination of mill shell rotation and gravity. Typically the GTBM's contain lifters, which on rotation of the shell inhibit sliding of the balls along the mill wall. In the GTBM, ball-powder interaction is dependent on the drop height of the balls.
The present method is distinguished from prior use of GTBM apparatus to grind flake, particles of foil, or other particles so as to reduce the particle size, and thereby to reduce the interparticle spacing of dispersoid. The present process differs from prior art grinding in a GTBM, for example, in the type of environment used in the mill, the time to achieve the end purpose and the type of product obtained. In general, to grind the particles in a mill, the milling is carried out in a medium which encourages fracturing of the particles. To mechanically alloy the components of a system, repetitive welding and fracturing of the particles are required. To achieve the appropriate weld/fracture system required for mechanical alloying, the processing is essentially dry and a process control agent may be necessary. Such agents will vary with the materials being processed. The process control agent may also contribute to the composition, e.g., as a precursor of oxides and carbides.
Early experiments appeared to indicate that, while mechanical alloying could be achieved in a GTBM, such mills were not as satisfactory as attritors for producing the mechanically alloyed powder in that it took a considerably longer time to achieve the same processing level. U.S. Pat. No. 4,443,249 discloses an improved process for producing mechanically alloyed powders on a commercial scale. The present invention is a further improvement in producing mechanically alloyed powders, and it may also be carried out in a GTBM.
As indicated above, mechanical alloying has a potential for use with a vast number of systems. The principles disclosed herein are of general application, enabling one to process materials in a GTBM in a practical and commercial manner. However, the description below will be mainly with reference to obtaining mechanically alloyed powders of materials which are readily mechanically weldable. This may occur, for example, in preparing alloy compositions containing metals such as aluminum, magnesium, titanium, copper, lithium, chromium and/or tantalum in sufficient amount for their cold weldability to become a major factor in processing.
The selection of a particular composition will involve the ultimate use of the end product produced from the mechanically alloyed powder. In many instances target properties are proposed by design engineers. Then new materials are sought to meet the target properties. For example, in recent years considerable research efforts have been expanded to develop high strength, light weight, materials which would satisfy the demands of advanced design in aircraft, automotive, naval and electrical industries. It is known to increase the strength of metals by the use of certain additives which will form, for example, oxide dispersion strengthened, age hardened or solution hardened alloys. The use of any particular additives or combinations of them depend on the desired properties. While high strength is a key target property to meet, ultimately it is the combination of properties of the material which determines whether it will be useful for a particular end use. Other properties which are often of interest are ductility, density, corrosion resistance, fracture toughness, fatigue resistant to penetration, machinability and formability.
Composition is only one contributing factor to properties. Mechanical alloying is another, in that it enables the unique combination of materials. Still another determinative factor is the processing level of the mechanically alloyed powder. As indicated above, a characteristic feature of mechanically alloyed powder is the mutual interdispersion of the initial constituents within each particle. In a mechanically alloyed powder, each particle has substantially the same composition as the nominal composition of the alloy. The power processing level is the extent to which the individual constituents are commingled into composite particles and the extent to which the individual constituents are refined in size. The mechanically alloyed powder can be overprocessed as well as underprocessed. An acceptable processing level is the extent of mechanical alloying required in the powder. It is one criterion in determining whether the the resultant powder product is capable of fulfilling its predetermined potential in respect to microstructural, mechanical and physical property requirements. Both underprocessed and overprocessed powders are not readily amenable to conversion to materials with the predetermined desired properties. Underprocessed powder has not been milled sufficiently long for the particles to be uniform or homogeneous with respect to the chemical composition and/or for the process control agent to be thoroughly interspersed in or react with the particles. Also, process control agents may become lost to the alloy composition, e.g. by evaporation, if not utilized at a time when the powders are exposed. In overprocessed powders the morphology of the powder may be sufficiently changed so as to make it more difficult to obtain the desired properties in the consolidated end product. In any event, for practical and economic reasons it is desirable to minimize milling time so long as the processing level achieved is acceptable. Processing beyond complete process control agent utilization may only add redundant cold work to the powder. Determination of the properties of a material can only be made after consolidation and thermomechanical processing of the powders. It will be appreciated that it is costly to learn at such a late stage that powder has not been processed to an acceptable level. Costs, inconvenience, loss of time and availability of equipment increase as the quantities of material increase. Thus, in a ball mill in which large amounts of high quality, high cost materials, such costs can make the materials unacceptable from an economic vantagepoint.
The present method offers a simple, economical way of meeting an acceptable processing level in mechanically alloyed powders.